Implement external services and handshake mechanism

[oliver-giersch]

Currently, all services must be "internal", i.e. they are explicitly started by the configuration system and run on the host machine. In order to realize "external" services running on distinct devices, such as an tablet, we need a concept for configuring and connecting such services as well.

Tasks

• add (basic) configuration syntax for declaring external services
• implement a handshake mechanism between reconf and external services
  • idea: services connects actively, reconf validates against expectations, then sends config message(s)
• implement a simple modular proof-of-concept service, e.g. an CLI app that acts as an alternative switch/control for all lamps and blinders